JRE means Java Runtime Environment :{, thanks to Rautamiekka. With it you can execute Java programs, but you can’t use it to make Java programs.
You should tell us that you are on a computer running some version of Windows (exact version, like XP, Vista, 8 or 10 required). I assume that you aren’t on Linux or Apple. If you are, then tell us so.
The thing to remember here is that nowadays computers can process data in chunks of 32 or 64 information units (bits), and programs that work together need to work with the same amount, so both should work with 32 bits, or both with 64. So as you have LibreOffice 64 bits running on a Windows version with probably 32 bits Java, LibreOffice won’t be able to use that version of Java. In fact, to make sure you don’t mess up your computer big time, Libre is made so it can’t even find a JRE made for the wrong amounts of bits. You will need to go to the Java website and download a 64 bits version. I found that was rather tricky, if you’re a novice, you may want to ask a friend to help you.
EDIT: When Java has been properly installed, open LibreOffice, select Tools - Options - LibreOffice - Advanced, tick Use a Java runtime environment and select the version you want to use in the box below by clicking the open circle at the far left on the same line as that version.
in it.